The site founder, pg, asked for suggestions on how to "stave off decline of HN" in a thread he opened 233 days ago
http://news.ycombinator.com/item?id=2403696
He then announced an experiment with the current practice of not showing comment karma scores in general 224 days ago,
http://news.ycombinator.com/item?id=2434333
noting that he might change settings back and forth as the experiment continued. About two days later he opened a simple yes/no poll about the experiment
http://news.ycombinator.com/item?id=2445039
and 216 days ago he replied to a question with a preliminary report on how voting behavior has changed.
http://news.ycombinator.com/item?id=2465271
As I recall, and as I just tried to verify by using HN search, the site founder pg and most of the other high-karma, long-time users here are reasonably well pleased with the results of not showing comment scores except to the participant who posts each comment.